The present invention generally relates to semiconductor processing, and in particular to a system and method for optimal development of a photoresist material layer on a wafer.
In the semiconductor industry, there is a continuing trend toward higher device densities. To achieve these high densities there has been and continues to be efforts toward scaling down device dimensions (e.g., at submicron levels) on semiconductor wafers. In order to accomplish such high device packing density, smaller and smaller features sizes are required. This may include the width and spacing of interconnecting lines, spacing and diameter of contact holes, and the surface geometry such as corners and edges of various features.
The requirement of small features with close spacing between adjacent features requires high resolution photolithographic processes. In general, lithography refers to processes for pattern transfer between various media. It is a technique used for integrated circuit fabrication in which a silicon structure is coated uniformly with a radiation-sensitive film, the resist, and an exposing source (such as optical light, x-rays, or an electron beam) illuminates selected areas of the surface through an intervening master template, the mask, for a particular pattern. The lithographic coating is generally a radiation-sensitive coating suitable for receiving a projected image of the subject pattern. Once the image is projected, it is indelibly formed in the coating. The projected image may be either a negative or a positive image of the subject pattern. Exposure of the coating through a photomask causes the image area to become either more or less soluble (depending on the coating) in a particular solvent developer. The more soluble areas are removed in the developing process to leave the pattern image in the coating as less soluble polymer.
Due to the extremely fine patterns which are exposed on the photoresist material, thickness uniformity of the photoresist material is a significant factor in achieving desired critical dimensions. The photoresist material should be applied such that a uniform thickness is maintained in order to ensure uniformity and quality of the photoresist material layer. The photoresist material layer thickness typically is in the range of 0.1 to 3.0 microns. Good resist thickness control is highly desired, and typically variances in thickness should be less than xc2x110-20 xc3x85 across the wafer. Very slight variations in the photoresist material thickness may greatly affect the end result after the photoresist material is exposed by radiation and the exposed portions removed.
Application of the resist onto the wafer is typically accomplished by using a spin coater. The spin coater is essentially a vacuum chuck rotated by a motor. The wafer is vacuum held onto the spin chuck. Typically, a nozzle supplies a predetermined amount of resist to a center area of the wafer. The wafer is then accelerated to and rotated at a certain speed, and centrifugal forces exerted on the resist cause the resist to disperse over the whole surface of the wafer. The resist thickness obtained from a spin coating process is dependent on the viscosity of the resist material, spin speed, the temperature of the resist and temperature of the wafer.
After the resist is spin coated and selectively irradiated to define a predetermined pattern, the irradiated or nonirradiated portions are removed by applying a developer material. The developer material is also spin coated onto the wafer by applying developer material across the resist and then spin coating the developer material until centrifugal forces disperse the developer material over the coating of resist. Due to the surface of the photoresist material layer on the semiconductor being highly hydrophobic, the surface can repel the developer material at the initial state of jetting out the developer material from the developer supply nozzle so that turbulent flow of the developer material is generated on the surface of the resist forming bubbles. The bubbles produced between the photoresist material layer and the developer material are a cause of defects in the resist pattern. Additionally, due to the developer being spincoated along a central point of the photoresist, the developer is not always uniformly applied across the photoresist material. This non-uniform distribution of developer can result in semiconductor defects.
Moreover, non-uniform distribution of developer causes problems related to critical dimension (CD) control. In particular, non-uniform distribution of developer across the photoresist means that substrates (typically, wafers or masks) have locations of different CD control. One must therefore consider these differences when attempting to optimize CD control, thereby compromising CD control quality in certain areas of the substrate.
After the photoresist material layer has been developed, the irradiated or nonirradiated portions are removed by rinsing or washing with a washing solution material. Each time a photoresist material layer is to be developed, a developer nozzle moves to the center of the photoresist material layer and applies the developer material. The developer nozzle then moves to the rest position and a washing solution nozzle moves above the wafer to rinse the developed portions and the developer material off the photoresist material layer. This constant movement of the different nozzles not only takes up a great deal of time, but eventually leads to mechanical problems and increased maintenance.
A prior art developer nozzle and washing solution application system is illustrated in FIGS. 1a-1b. A multiple tip developer nozzle 10 is coupled to a pivotable arm 12 that pivots from a rest position to an operating position. In the operating position, the multiple tip nozzle 10 applies a developer material 26 on a resist layer 24 disposed on a wafer 22. The wafer 22 is vacuum held onto a rotating chuck 20 driven by a shaft 18 coupled to a motor 16. The developer material flows outward from the center of the photoresist material layer 24 covering the entire top surface of the photoresist material layer 24. A washing solution nozzle 28 is coupled to an arm 32 and moves from an operating position to a rest position. The washing solution nozzle provides a washing solution material 30 to rinse the developed photoresist and the developer material from the photoresist material layer 24. As illustrated in FIG. 1a, the washing solution nozzle 28 is typically at a much greater distance from the photoresist material layer in its operating state than the developer nozzle is when it is in its operating state resulting in a splashing effect that can scatter particles and cause defects.
In view of the above, there is an unmet need for a system/method for dispensing a uniform layer of developer across a photoresist material layer formed on a wafer. There is also and unmet need for a system/method that provides a rinse that mitigates splashback during rinsing of the developed photoresist and developer material from a photoresist material layer.
The present invention provides a system and method of applying a developer to a photoresist material layer disposed on a semiconductor substrate. The developer system and method employ a developer plate having a plurality of application apertures for dispensing developer and a plurality of exit apertures for allowing excess developer to flow out from between the developer plate and the photoresist material layer. The exit apertures provide better control of the rate of application of the developer as well as the application of any washing solution being employed in the development process.
Preferably, the developer plate has a bottom surface with a shape that is similar to the wafer. The developer plate is disposed above the wafer and substantially and/or completely surrounds the top surface of the wafer during application of the developer. A small gap is formed between the wafer and the bottom surface of the developer plate. A small gap is defined as a gap having a size from about 0.5 to about 5 mm. The wafer and the developer plate form a parallel plate pair, such that the gap can be made small enough so that the developer fluid quickly fills the gap. The developer plate is disposed in very close proximity with respect to the wafer, such that the developer is squeezed between the two plates thereby spreading evenly the developer over the wafer. Excess material that flows through the exit apertures can be removed by a vacuum system or the like.
Preferably, the developer plate and the wafer are rotated in the same direction at the same speed or frequency so that the amount of agitation can be controlled to strictly a radial mode. Alternatively, the developer plate and the wafer can be rotated in the same direction at different speeds and frequencies to increase the agitation of the developer. Furthermore, the developer plate and the wafer can be rotated in different directions at the same or different speeds and frequencies to increase the agitation of the developer.
Moreover, the proximity of the developer plate to the wafer during application and the size of a plurality of apertures in the developer plate provides for improved localization with respect to development of the photoresist material layer. Since very little surface area of the photoresist material layer is exposed, evaporation rates can be minimized with respect to conventional development, thus improving temperature control. Additional improvements in temperature control can be obtained by heating the developer plate. In one aspect of the invention, the developer plate is also provided with a washing or rinsing solution for washing or rinsing the developed photoresist from the wafer. The developer plate can include separate apertures and supply mechanisms for supplying the washing solution to isolate the developer from the washing solution. Since the wafer is covered during spin rinsing, splashback effects are minimized.
One aspect of improved localization with respect to development of the photoresist material layer involves better CD control. Improved CD control is obtainable employing the present invention since the developer is dispensed and spread relatively equally over the photoresist surface. That is, substantially the same CD control is achieved at various locations across the photoresist surface.